1. Field of the Invention
The present invention relates to production of fluorine doped tin oxide coatings using a liquid precursor. Such coatings may be used, for example, as heat reflective elements in double glazed windows.
2. Description of the Background Information
A precursor of the above type has been proposed in European Patent Document EP-A-0 158 399. This patent discusses the prior art and its applications in some detail along with the different processes used to produce such coatings, such as solution spraying, chemical vapor deposition, powder deposition, as well as the properties of the coatings including particular low sheet resistance and high visible light transmissivity. This document particularly mentions the drawbacks of known techniques such as the use of organic solvents which must be eliminated (Japanese Application 75-61415), difficulties in controlling the flow rate of the gaseous stream (EP-A-0 112 780), the danger of explosion and the toxic nature of the materials employed (U.S. Pat. No. 4,265,974), or the necessary use of high temperatures when using a solid precursor (U.S. Pat. No. 4,293,594).
Apart from the difficulties associated with the prior art processes, EP-A-0 158 399 emphasizes that, as taught in Japanese Application 75-61415 and EP-A-0 112 780, the electrical properties of the coatings obtained are not always satisfactory, and that generally they do not possess optimal transparency.
In order to overcome these drawbacks, EP-A-0 158 399 proposes a liquid composition including a mixture comprising (1), as the doping compound, an organic fluorine compound selected from trifluoroacetic acid, trifluoroacetic anhydride, ethyl trifluoroacetoacetate, trifluoroethanol, ethyl trifluoroacetate and pentafluoropropionic acid, (2) an organotin derivative selected from an alkyltin trichloride, a dialkyltin dichloride, an alkyldichlorotin acetate, a dialkyldichlorotin diacetate, a dialkylchlorotin acetate, an ester or tin tri- or tetrachloride, and (3) optionally a polar organic derivative whose function is to keep the composition stable and in a single phase at temperatures below the ambient temperature.
In the absence of the polar organic derivative, which is for example methylisobutylketone, acetone anhydride or ethyl acetate, constituents (1) and (2) are not miscible when cold which makes homogenous deposition difficult. Presence of the polar organic derivative renders the mixture homogeneous but inflammable, with a reduced deposition rate. In addition, pyrolysis yields are relatively low which has the drawback of producing a large quantity of solid residues containing tin oxide in the fume evacuation system.
Another prior art document, U.S. Pat. No. 4,857,095, proposes the use of a solution containing dibutoxydibutyltin and trifluoroacetic acid to produce a fluorine doped tin oxide layer. While the patent indicates that a very transparent coating is obtained with very low turbidity and no cloudiness, in fact, when the technique in question is employed, many white deposits are observed on the substrate glass and in the fume evacuation system.
JP-A-62 70247 (reported in Chemical Abstracts, Vol. 107, No. 6, Aug. 10, 1987, Columbus, Ohio, USA) teaches a composite solution made from:
a solution of a tin compound containing fluorine, such as a solution of dibutyltin (or dimethyltin) di(trifluoroacetate) or butyltin tri(triflouroacetate), and PA1 a solution of a tin compound containing chlorine, such as a solution of trichlorobutyltin or tetrachlorotin. PA1 60 to 90% by weight of at least one chlorinated organotin derivative, wherein the tin atom is bound to at least one chlorine atom and to at least one carbon atom of a hydrocarbon chain; PA1 5 to 30% by weight of at least one organotin mono(fluoroalkanoate) wherein the tin atom is bound to at least one carbon atom of a hydrocarbon chain; and PA1 0 to 15% by weight of a tetraorganotin compound wherein the tin atom is bound to four carbon atoms each forming part of a hydrocarbon chain.
The solution is sprayed on heated glass substrates to form a fluorine doped SnO.sub.2 coating having a low resistivity to infrared rays.
In all of the examples of this prior art document, the tin compounds containing fluorine and chlorine are in solution in trichloroethane. This solvent, like other chlorinated solvents, is dangerous and toxic.
Despite, as shown by the state of the art, a great deal of work on production of fluorine doped tin oxide coatings on substrates, there remains a need for a precursor which can produce fluorine doped tin oxide coatings of high quality, i.e., having a regular thickness and without blemishes, which can be deposited at the high speeds used in continuously coating flat "float" glass, without using an inflammable organic solvent and ending with products having low surface electrical resistance and a high light transmission coefficient.
It has now been discovered that by appropriately selecting the tin compounds containing chlorine and flourine, one obtains a precursor which, upon evaporation, provides a homogeneous phase whose quantitative and qualitative composition corresponds to that of the precursor. It then becomes possible to deposit this precursor during the vapor phase.
Such a vapor phase deposit is not possible in the case of the mixtures described by JP-A-62 70247 because, if they were subjected to the conditions of a vapor phase deposition, their solvent CCl.sub.3 Me would evaporate before the di- or tri(fluoroalkanoates). This is why JP-A-62 70247 resorts to a liquid pyrolysis.
The vapor phase deposition has considerable advantages resulting from the absence of the solvent, which, as indicated above, not only has a harmful effect on health and the environment, but also detrimentally affects the deposition yield. Specifically, the presence of this solvent necessarily reduces the quantity of tin by unit of volume of the precursor, and it follows that the deposition rate of the fluorine doped tin layer on the glass is consequently reduced. Deposition by liquid pyrolysis is therefore less adapted to a continuous deposition on float glass than vapor phase deposition.